Botulism

Humans are susceptible to the A, B, E and F toxins. Farm animals and (water)fowl are susceptible primarily to the C and D toxins, while the B-toxin is found in horses now and then . In fish and piscivorous birds, type E can be fatal. Dogs are much less susceptible to the botulinum toxins, but botulism may occur in rare occaisions.

Toxin types F and G do not occur in the Netherlands as far as we know.

Botulism can lead to mass die-off among waterfowl. C. botulinum occurs naturally in the environment inhabited by waterfowl. As a result, the bacteria also enter the birds' gastrointestinal tracts. Under normal circumstances, this is not a problem; however, if the bird dies, and the outside temperature rises above 25° C for some time, then C. botulinum can replicate in the carcass. The larvae of insects feeding on the carcass take up these toxins. These larvae are not susceptible, but may carry extremely high concentrations of the toxins. These larvae (small toxin bombs) are then fed on by waterfowl, with the result that they die of botulism. A vicious circle arises in this way, with waterfowl dying off in large numbers if ther is no intervention. This is also the most important reason for mainly dead waterfowl to be cleared, in order to prevent large-scale outbreaks.

How is botulism recognised in waterfowl?

The most visible symptom in waterfowl is paralysis of the neck and wing muscles, often followed by death. A large number of waterfowl dying and observation of symptoms of paralysis during or shortly after a warm period are strong indications of botulism. Poisoning may proceed so rapidly, as a result of the high concentration of toxins, that no birds are found alive.

The treatment of animals with botulism consists in stopping use of the current feed (and destroying this feed) and going over to feed that is uncontaminated with botulism toxins. Secondly, the animal's symptoms must be treated by maintaining the moisture and energy balance. Antiserum may be administered to bind the toxins present in the bloodstream; this makes sense only during the acute phase of the disease just after the symptoms first show.

The antiserum is not registered as an animal medication and falls under the cascade provision for animal medicines (Art. 22, Section 1 of the Veterinary Medicines Decree (Diergeneesmiddelenbesluit)). Antiserum against the toxins of type A, C, D and E is available through a vet from Wageningen Bioveterinary Research (WBVR). An evaluation is made in consultation with a WBVR expert on whether treatment with antiserum is appropriate. Stocks of antiserum are limited, and it is not intended for the treatment of entire populations.

If botulism occurs in waterfowl, the dead birds must be removed as quickly as possible to prevent these animals transmitting botulism to others. Dead waterfowl can be reported to the manager of the water body where the animal was found, usually the water authority or the municipality (with larger bodies of water, usually Rijkswaterstaat). They will have the animals removed and tested for botulism. Apart from removing carcasses, ensuring adequate water flow is advised. This leads to a rise in the oxygen content, a decline in the water temperature and a lowering in the concentration of spores and toxins. Extremely high concentrations of toxins are often present in carcasses. For this reason, precautionary measures should be taken on contact with dead animals, such as wearing protective (disposable) gloves.